Reconfigurable scanner and RFID system using the scanner

ABSTRACT

A scanner has plasma loop or plasma window antennas for selectively scanning for ID tags along distinct radials of the scanner. Scanner elements are made electromagnetically invisible to adjacent elements by removing power or lowering plasma densities so that the scanner elements do not interfere with its own operation. Activatable ID tags and a shipping container suitable for scanning with electromagnetic energy are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Pat. No. 6,700,544application Ser. No. 10/067,715 filed Feb. 5, 2002, the entirety ofwhich is hereby incorporated by reference. This application is also acontinuation-in-part of U.S. Pat. No. 6,870,517 application Ser. No.10/648,878 filed Aug. 27, 2003, the entirety of which is herebyincorporated by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates generally to the field of RFID (radiofrequency identification) and in particular to a new and usefulplasma-based sensor array used to detect the presence of an interactiveelement resulting from interaction of antennas having variableconductive sections by magnetic induction and/or electromagnetic waves.

RFID systems have gained much popularity recently as a means forwireless tracking of individual objects for a variety of purposes. Forexample, some retailers have proposed using unique RFID tags attached toproducts they sell to be able to track each piece from the distributionwarehouse to the store shelves, and potentially, to customer's home.RFID systems have applications in anti-theft, product marketing,intelligence gathering, and security systems, among others.

Near-field readers incorporating sensors and identification tags aregenerally known for use in scanning systems. As used herein, near fieldsexist at distances ranging from a fraction of a millimeter to a fewmiles, depending on frequency. The near field is defined as when thewavenumber times the distance of the range of the antenna is less thanone. The far field is defined as when the wavenumber times the distanceof the range of the antenna is greater than one. The wavenumber is 2π/λ.

Near-field reader systems take advantage of magnetic field interferencebetween a powered transceiver and a powered or passive object to detectthe presence of the object by receiving a return signal from the objectwith the transceiver.

Presently, card and label near-field readers are formed by metal loopswhich read data in the near electromagnetic field. In the near-fieldsituation, for a loop antenna, the electric field is effectively zeroand only the magnetic field is present. Thus, near field loop antennasuse mutual inductance between active and passive loop antennas to causethe active loop antenna to receive data from the passive loop antenna.That is, the magnetic flux from one loop antenna induces a current in asecond loop antenna having properties dependent on the current andvoltage in the first loop. The magnetic flux interaction and inducedcurrent can be used to transmit information between the loop antennasbecause of the dependency. The near-field loop antennas can be morecorrectly considered loop sensors or loop readers, since there is noelectric field interaction between the active source and a passive loop.

RFID systems, in contrast, can be both near and far field devices. RFIDsystems generally have a longer range than most near-field systems,because they use radio frequencies, such as 900 MHz, 2.4 GHz, and, morerecently, 5.8 GHz to transmit and receive information between sensorunits and passive ID tags.

A problem with all metal antennas used in a sensing array is that evenwhen they are not active, several antennas arranged in a multipleorientation array still create unavoidable mutual inductance andelectromagnetic wave interferences between antennas. That is, even ifthe metal antenna sensors in an array are sequentially activated, theystill cause mutual interference with other ones of the antennas. Theinterferences result in detuning of the antennas in the array, so thatspecial considerations must be made when forming arrays of metalantennas.

In the case of inductive loop antennas, to optimize the strength of themutual inductance field between an active loop sensor and a passive loopantenna, the antennas must be parallel to each other. If the antennasare perpendicular, the magnetic field is zero at the passive loop andthere is no mutual induction. The strength of the magnetic field at thepassive loop increases as the loops move from a perpendicular to aparallel orientation. For a device to effectively scan a region for apassive loop, a single loop must move through a variety of orientations.The range of effectiveness of an antenna is based on the orientation ofthe passive and active loops to each other and the diameter of the loopof the active sensor.

Patents describing scanning antenna systems using interaction betweenactive and passive antennas include U.S. Pat. No. 3,707,711, whichdiscloses an electronic surveillance system. The patent generallydescribes a type of electronic interrogation system having a transmitterfor sending energy to a passive label, which processes the energy andretransmits the modified energy as a reply signal to a receiver. Thesystem includes a passive antenna label attached to goods that interactswith transmitters, such as at a security gate, when it is in closeproximity to the transmitters. The label has a circuit which processesthe two distinct transmitted signals from two separate transmitters toproduce a third distinct reply signal. A receiver picks up the replysignal and indicates that the label has passed the transmitters, such asby sounding an alarm.

U.S. Pat. No. 3,852,755 teaches a transponder which can be used as anidentification tag in an interrogation system. An identification tag canbe encoded using a diode circuit in which some diodes are disabled toproduce a unique code. When the identification tag is interrogated by atransponder, energy from the transponder signal activates the electroniccircuit in the tag and the code in the diode circuit is transmitted fromthe tag using dipole antennas. The transponder uses a range offrequencies to send a sufficiently strong signal to activate a nearbyidentification tag.

A vehicle identification transponder using high and low frequencytransmissions is disclosed by U.S. Pat. No. 4,873,531. A transmittingantenna broadcasts both high and low frequency signals that are receivedthrough longitudinal slots in a transponder waveguide. Transverse pairsin the waveguide adjacent the longitudinal slots indicate a digital “1”,while the absence of transverse pairs produces a digital “0”. The highand low frequencies are radiated from the transverse pairs to high andlow frequency receiving antennas. The transmitting and receivingantennas are fixed relative to each other and move with respect to thetransponder.

U.S. Pat. No. 5,465,099 teaches a passive loop antenna used in adetection system. The antenna has a dipole for receiving signals, adiode for changing the frequency of the received signal and a loopantenna for transmitting the frequency-altered signal. The originaltransmission frequency is changed to a harmonic frequency by the diode.

As discussed above, near-field loop sensors or readers differ from farfield loop antennas by the basic difference that in the near-field, theelectric field is usually very small and the magnetic field of anelectromagnetic radiant source is controlling, while in the far field,the interaction is via electromagnetic waves. As will be appreciated,the relationships between sources and receivers are different as welldue to the different distances and fields which affect communicationbetween them.

Plasma antennas are a type of antenna known for use in far fieldapplications. Plasma antennas generally comprise a chamber in which agas is ionized to form plasma. The plasma radiates at a frequencydictated by characteristics of the chamber and excitation energy, amongother elements.

Plasma antennas and their far field applications are disclosed inpatents like U.S. Pat. Nos. 5,963,169, 6,118,407 and 6,087,992 amongothers. Known applications using plasma antennas rely upon thecharacteristics of electromagnetic waves generated by the plasma antennain far field situations, rather than magnetic fields in near-fieldconditions.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a scanning sensorarray which eliminates interference between adjacent sensors in thearray in both near-field and far-field application environments.

It is a further object of the invention to provide a scanning readerarray which can be arranged to scan in multiple directions withoutconcern for interference between array components.

Yet another object of the invention is to provide a scanning arraycomposed of variable conductive elements.

A still further object of the invention is to provide an apparatus andmethod for scanning a volume for an interactive component containing adata using a reader with variable conductive elements.

Accordingly, an scanner using antennas is provided which effectivelyscans for items having readable data sources in controllable directionswithout interference between scanner components. The scanner transceivessignals by magnetic induction or electromagnetic wave interaction alongsequentially selected radials using antennas formed using variableconductive elements.

The scanner is provided in two embodiments. In a first embodiment, anarray of plasma loop sensors are sequentially made active to scan aspace to identify an interactive object comprising a data source basedon mutual inductance or electromagnetic wave interaction of the scanningplasma reader with the data source. The data source can be an active orpassive antenna of any type, including loop antennas. The plasma loopsensors are variable conductive elements, in that they are conductingonly when powered.

The array of plasma loop sensors are connected to a power source, whichmay include a frequency switching circuit, and to a sensor circuit. Thepower source provides power to each of the plasma loop sensors asdetermined by a sequential switch circuit to make the loop sensorsactive in turn. The sensor circuit is used to interpret signals receivedfrom the data source by each plasma loop sensor while it is active.

One or more plasma loop readers can be arranged in arrays in differentorientations to form a sensor and then sequentially activated tosimulate a change in orientation of the sensor without any physicalmovement of the plasma loops in the array. Since the inactive plasmaloop sensors are effectively electromagnetically invisible to the activeplasma loop reader, there is no interference created between them. Thatis, so long as at least a section of the loop is formed by a plasmatube, the loop will be electromagnetically invisible to other sensorloops. When the loop has at least a section is plasma, the remainder maybe another conductor, such as metal.

The plasma loops can be activated and deactivated in microseconds, sothat very rapid switching among several plasma loops is possible. Theplasma loop readers in the sensor can be arranged in a variety ofconfigurations, including a sphere, a cylinder or other geometric shape.The terminals of each plasma loop reader in the configuration areconnected to the power source via a switching circuit and to the sensorcircuit.

In a further embodiment of the plasma loop readers, they may haveseveral loops of different diameter joined at a common side. That is,there is a common area at the terminals where a portion of thecircumference of each loop is the same. When a frequency switch is usedin connection with the power source, the power frequency used toactivate the plasma loops can be varied to change the frequency at whichthe plasma loop reader is active. The particular diameter loop in whichthe plasma is active in the plasma loop sensor is also changed bychanging the active transmission frequency.

In yet another alternative of the plasma reader, the plasma loops arereplaced by metal loops with sections of plasma loop which can be turnedon and off. The plasma loop sections, or plasma switches, aresufficiently large so that when they are turned off, or made inactive,the metal loop is opened enough that it rendered electromagneticallyinvisible and no longer interferes with any surrounding active loopreaders. The plasma loop sections are connected to the power source inthe same manner as the full loops and can be switched in the same way.

In a still further alternative, plasma loop sections may be combinedwith metal loop sections and mechanical switches, such as relays andsolid state devices. The metal loop sections may form up to a length ofthe loop which is effectively electromagnetically invisible when theswitch is used to deactivate the loop.

It is intended that the sensor circuit connected to the antennas in thearray will be capable of interpreting data received from existing typesof passive loops commonly used in security devices and the like. Theplasma loop sensor interacts with existing passive loops in the samemanner as metal loop sensors, but does not suffer from detuning orinterference from surrounding loop sensors.

In a second embodiment of the scanner, a steerable antenna is providedcombining a transceiving antenna with one or more arrays of variableconductive elements for filtering, phase shifting, steering, polarizing,propagating and deflecting an incident signal at non-backscatteringangles.

One embodiment of the steerable antenna comprises an antenna having aswitchable electromagnetic shield of variably conductive elements forcontrollably opening a transmission window at selected radial anglespositioned at an effective distance to intersect at least thetransmission radials for the antenna. Preferably, the antenna isomnidirectional and the shield is concentric around the antenna tointersect all transmission radials for the antenna. The shield may alsoinclude switchable variable conductive elements for controlling anelevation angle of the transmission lobe passing through the window, sothat the antenna is steerable on two axes.

The electromagnetic shield is formed by a cylindrical annular ring ofswitched variable conductive elements. In one embodiment, the shield isa ring of plasma tubes extending parallel with the omnidirectionalantenna. Alternately, when transceiving in appropriate frequency ranges,the shield is a ring of photonic bandgap crystal elements orsemiconductor elements. When the variable conductive elements arenon-conducting or at low density in the case of plasma, so that theplasma frequency is lower than the incident transceived frequencies, thevariable conductive elements are off and form a transmission window. Theomnidirectional antenna can be a conventional metal dipole or otherconfiguration antenna, a plasma antenna or an optical wavelengthtransmitter. Plasma antennas include nested plasma antennas and evenstacked plasma arrays of the same type used to form the shield.

The transmission window is formed by either turning off power to theappropriate electromagnetic shield elements, or otherwise making thedesired shield elements transparent to the transmitting antenna, such asby reducing plasma density below the threshhold needed to blocktransmission of an incident signal frequency. The shield elements arepreferably rapidly switchable, so that the radial transmission directionof the antenna can be changed within microseconds, or faster byPerot-Etalon effects. The shield elements are selected for use withantennas broadcasting on a broad range of frequencies includingmicrowave to millimeter range (kHz to GHz), TeraHertz, infrared andoptical ranges.

An alternate embodiment of the shield utilizes a cylindrical array ofswitchable variable conductive elements to provide more selectivecontrol over where openings in the shield are formed. The cylindricalannular shield with the array surrounds an antenna. The elements formingthe array are arranged in multiple rows and columns on a substrate. Thesubstrate can be a planar sheet rolled into a cylinder shape. Thevariable conductive elements can be either switchable regionssurrounding air or other dielectrics in fixed gaps or slots, so that theeffective size of the fixed slots can be changed rapidly, or theelements can be formed as linear conductors, rectangles, stars, crossesor other geometric shapes of plasma tubes, photonic bandgap crystals orsolid state semiconductors on the substrate. The substrate is preferablya dielectric, but may also be made from a conductive metal.

A more complex shield for the antenna has one or more stacked layers,with each layer being a switchable array of variable conductiveelements. The layers are spaced within one wavelength of adjacent layersto ensure proper function. Each switchable array in the stack can be afilter, a polarizer or a phase shifter, a deflector, or a propagatingantenna. The layers are combined to produce a particular effect, such asproducing a steerable antenna transmitting only polarized signals inspecific frequency bands.

Layers of annular rings, for example, can be stacked at distancescorresponding to wavenumber times distance from the central antennawhich correspond to transmission peaks for particular frequencies. Bystacking several frequency-selective layers, a multi-frequency antennais produced which is controllable to selectively transmit and/or receiveeach frequency along a particular radial of the antenna.

In a further embodiment of the invention, the scanner can be used totrack a particular ID tag when one or both are moving, without physicalre-orientation of the scanner. A central unit can be stationary ormobile and has a scanner with one of the two antenna configurationsdescribed which is controllable to scan along a specified radial fromthe scanner. The central unit includes circuits for determining when aconnection is made between the scanner and ID tag and maintaining theconnection while they move relative to each other. Once a connection ismade, the electromagnetic shield of the satellite unit steerable antennais activated to produce only a transmission window and radiation lobealong the radial axis needed to maintain the connection with the centralunit. The steerable antenna shield on the central and each connectedsatellite unit is adjusted to compensate for their relative movementwhile maintaining the connections.

Conventional ID tags made of metal which are either passive or activelytransmit can be used with the scanner of the invention. An ID tag havinga variable conductive element forming the tag antenna is provided aswell.

The ID tag with variable conductive element antenna can be an activetransmitting or a passive transmitting antenna. Further, the ID tag canhave an active variable conductive element or a passive variableconductive element. That is, the antenna is a plasma element which iseither connected to an active transmitter, or does not transmit anyinformation and is only sensed by electromagnetic interference. And, theplasma element can be normally powered and active and capable of beingsensed by a scanner, or inactive and thus, electromagneticallyinvisible. The antenna can be normally inactive, but weakly or partiallyionized and made active by exciting the plasma element to an activeenergy state is provided as well.

The inactive plasma element is excitable to an active state by anincident received signal. The plasma is energized and permits the ID tagto generate a detectable return signal with date or interference inresponse to the incident signal. The incident signal may be a scanningsignal or other energizing signal. The plasma in the plasma element maybe maintained in a weakly or weakly partially ionized state by a powersource, such as a battery, laser, voltage source, a radiation source orradioactive source in a known manner, so that the plasma is more easilyfully energized by the incident signal.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference is made to the accompanying drawings and descriptive matter inwhich a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1A is a front elevation view of a plasma loop antenna of theinvention;

FIG. 1B is a front elevation view of an alternative plasma loop sensoraccording to the invention;

FIG. 2 is a side elevation view diagram of the magnetic fieldinteraction between a plasma loop sensor of FIG. 1 and a passive loop;

FIG. 3 is a diagram of an array of plasma loop readers at differentorientations;

FIG. 4 is a schematic diagram of a transceiver circuit for use with aplasma sensor system;

FIG. 5A is a front elevation view of a metal loop sensor with a plasmasection;

FIG. 5B is a front elevation view of an alternative embodiment of themetal loop sensor and plasma section of FIG. 5A;

FIG. 5C is a front elevation view of a second alternative embodiment ofthe metal loop sensor and plasma section of FIG. 5A;

FIG. 5D is a front elevation view of a third alternate embodiment of aloop having metal and plasma sections and a switch;

FIG. 6 is a front perspective view of an array of plasma loop readersmounted in a spherical substrate;

FIG. 7 is a sectional top plan view of an alternative embodiment of thearray of FIG. 6 taken across an equator of the spherical substrate;

FIG. 8 is a front perspective view of a cylindrical substrate holding anarray of plasma loop sensors;

FIG. 9 is a top plan view diagram of a grocery or department storecheckout using a plasma loop sensor array of the invention;

FIG. 10 is a side elevation view of a diagram of a toll collectionsystem using plasma loop arrays according to the invention;

FIG: 11 is a front perspective view diagram of a security gate systemusing a plasma loop scanning array according to the invention;

FIG. 12 is a top, left, front perspective view of a cube having a sensorloop on each of the three faces adjacent a vertex;

FIG. 13A is a schematic representation of a planar array of variableconductive elements on a dielectric surface in a non-conducting state;

FIG. 13B is a schematic representation of a planar array of slotelements on a dielectric surface in a non-conducting state;

FIG. 13C is a schematic representation of a polarizer in the form of aplanar array of spoked variable conductive elements on a dielectricsurface in a non-conducting state;

FIG. 13D is a schematic representation of a planar array ofprogressively sized, variable conductive elements on a dielectricsurface in a non-conducting state;

FIG. 14A is a schematic representation of an omnidirectional antennasurrounded by an annular plasma ring;

FIG. 14B is a diagram of an omnidirectional antenna surrounded by eightplasma tubes with seven energized;

FIG. 14C is a diagram of an omnidirectional antenna surrounded bysixteen plasma tubes with fifteen energized;

FIG. 15A is a top plan view of a omnidirectional antenna used withlayered arrays of the invention;

FIG. 15B is a side elevation view of the antenna configuration of FIG.6B;

FIG. 16 is a diagram illustrating the radiation pattern of a steerableantenna of the invention;

FIG. 17 is a diagram illustrating the radiation pattern for adifferently configured steerable antenna of the invention;

FIG. 18 is a diagram displaying electromagnetic wave interaction betweena scanning antenna and passive and active ID tags;

FIG. 19 is a diagram illustrating a scanner of the invention used todetermine the contents of a ship containing goods marked with ID tags.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, plasma loop sensor and plasma loop reader are intendedto both mean an active loop device having at least a section of plasmatube, as will be described further herein, when used in the near-field,and composed of only plasma tubes in far-field applications. The activeloop device is an electro-magnetic transducer having a conductive plasmasection. That is, the plasma loop reader or sensor can both generate amagnetic field or electromagnetic wave, depending on whether it is fornear or far-field applications, and sense a corresponding interferinginduction current or electromagnetic wave caused by a passive or activeloop within range of the reader or sensor.

The terms plasma tube or plasma loops referring to plasma elementsshould not be taken as limiting on the geometric shape generally definedby the stated shape, except when the shape is essential to the functionof the plasma element. Any linear dipole, traveling wave antenna, Yagiantenna, log periodic antenna, horn antenna, or aperture antenna can beused for the plasma loop antenna herein. Thus, the plasma element may beformed as a circular loop, a helix, a coil, an ellipse, a rectangle, aspiral or another shape suitable for emitting or receiving a signal.

Further, variable conductive element as used herein includes a plasmaelement, a photonic bandgap crystal, or a semiconductor, unlessotherwise specified.

Referring now to the drawings, in which like reference numerals are usedto refer to the same or similar elements, FIG. 1A shows a plasma loopsensor 10 primarily comprising a tube 12 having electrodes 25, 27 ateach end. The tube 12 is bent into a circular loop. A pair of leads 20,22 are attached to the electrodes 25, 27 for connecting the tube 12 to apower source (not shown in FIG. 1A).

The tube 12 of the plasma loop sensor 10 contains a gas 15 inside theplasma loop sensor 10. The gas 15 may be neon, xenon, argon or othernoble gases, as well as mercury or sodium vapors, or other materialsfound to produce a suitable plasma. The gas 15 can be ionized to form aplasma in the tube 12 by applying energy to the gas 15 using any ofseveral devices including electrodes 25, 27, inductive couplers,capacitive sleeves, lasers or RF heating.

When the gas 15 is ionized, a current I begins to flow between theelectrodes 25, 27, which in turn generates a magnetic field having amagnetic flux B. The magnetic field is generated in a directionperpendicular to the plane of the loop antenna 10. The magnetic field ischaracteristic of the current I and voltage used to power the plasma inthe tube 12.

The plasma loop sensor 10 optimal magnetic induction range is equal tothe radius r of the loop. The plasma loop sensors 10 may be made anysize as is practical and required by a particular application. Forpurposes of the invention herein, however, the preferred radius for theplasma loop antennas is between 0.5 cm and 100 cm. Further, it should benoted that although the optimal range of the plasma loop sensors 10 islimited by the radius of the loop, the sensors 10 are still effectiveacross a wider range of distances. The plasma loop sensors 10 may beswitched on and off in a matter of 1-10 microseconds, with rapid riseand decay times, so that very rapid switching of the plasma loop readers10 is possible.

The frequency of the ionization energy source also affects the plasmamagnetic field radiation frequency. It is possible for the sensors 10 toradiate at frequencies ranging from 0.1 MHz into the Terahertz range.

The plasma loop reader of FIG. 1B is a multiple loop plasma reader 71having three different diameter tubes 72, 73, 74 with a commontangential side 75 and electrodes 25, 27. A gas inside the tubes can beionized to different excitation levels depending on the energy appliedat the electrodes 25, 27. The different ionization levels correspond todifferent radiant frequencies for the electro-magnetic fields generatedby the plasma reader 71. Thus, the multiple loop plasma reader 71 can beused to generate multiple transmission frequencies or to receive ondifferent frequencies from transmission by changing the energy suppliedto the plasma loop reader 71.

FIG. 2 illustrates the interaction of a magnetic field 40 of a plasmaloop sensor 10 with a passive metal loop 35. Plasma loop sensor 10 has aplasma current of which generates magnetic field 40 around the loop 10.The magnetic field 40 is sufficiently strong to at least effectivelyextend a distance of about twice the radius r of the loop 10 to passiveloop 35. Magnetic field 40 induces a current I_(i) in the passive loop35.

Passive loop 35 includes a frequency changing circuit 36, which operateson induced current I_(i) to alter the frequency of the received magneticfield and produce a frequency-changed response magnetic field. Thefrequency changing circuit 36 causes the induced current I_(i) to havethe altered frequency. The circuit 36 may be connected to the terminalsof the passive loop 35 in a known manner. Passive loop 35 and frequencychanging circuits 36 known in the prior art disclosed herein, forexample, may be used for these components.

The induced current I_(I), with a different frequency from the plasmacurrent I_(A), generates a response magnetic field 45 emanating from thepassive loop 35. The response magnetic field 45 is also sufficientlystrong so as to interact with the plasma loop sensor 10. As describedfurther below, the plasma loop sensor 10 can also operate in a receivemode to detect response magnetic field 45. In the receive mode, theplasma loop sensor 10 has a second induced current that is differentfrom plasma current I_(A), with characteristics corresponding to theresponse magnetic field 45.

It should be noted that if the response magnetic field 45 is varied inresponse to a changing induced current I_(i) controlled by the frequencychanging circuit 36, that more complex communication is possible, suchas transmission of an identifying code in addition to simply indicatingthe presence of the passive loop 35.

When the plasma loop sensor 10 and passive loop 35 too far apart to takeadvantage of the near-field situation and magnetic induction isinsufficient to generate a response, the plasma loop sensor 10 can beused in a far-field type application instead. The plasma loop sensor 10can be configured to transmit an electromagnetic wave, which generates acorresponding response similar to the magnetic induction response in thepassive loop 35.

Thus, regardless of whether the interaction is only through magneticinduction or by electromagnetic wave, a single plasma loop sensor 10 canbe used to detect the presence of a passive loop 35 and receivecommunications therefrom. However, the ability of the plasma loop sensor10 to generate the induced current I_(i) so that a response magneticfield is subsequently generated and received is dependent in part on therelative orientation of the plasma loop sensor 10 and passive loop 35 toeach other. The loops 10, 35 must be oriented parallel to each other, asshown in FIG. 2, so that the interaction between the generated magneticfields 40, 45 is maximum. As the relative orientation between theantennas 10, 35 changes from parallel to perpendicular, the fieldinteraction with the antennas 10, 35 goes from maximum to zero.

To solve this problem, there are two primary solutions. One is tophysically move the loops 10, 35 relative to each other to coverdifferent orientations. The other is to create an array of severaldifferently oriented plasma loop sensors 10 that can be sequentiallyactivated to send and receive magnetic fields 40, 45.

In the latter case, plasma loop sensors 10 provide the benefit that theycan be easily switched on and off rapidly in sequence. Further, plasmaloop sensors 10 can be arranged in any type of sequentially-fired arraywithout affecting adjacent ones of the plasma loop sensors 10 becausewhen the gas 15 is not being ionized to form plasma, the inactive sensor10 is electromagnetically invisible to another, active plasma loopsensor 10.

An example of an array 100 is shown in FIG. 3, in which seven plasmaloop sensors 10 are arranged co-planar directed to different angles at30° intervals. Although the plasma loop sensors 10 are shown arranged inan arc, this is only for purposes of illustrating the rotation todifferent angles and is not required. The plasma loop sensors 10 may bearranged co-linear as well, with each loop sensor 10 being rotated 30°from the facing of the previous loop sensor 10. Further, the angularrotation from one antenna to the next may be more or less than 30°,depending on the number of plasma loop sensors 10 in the array 100 andthe desired effective range of each plasma loop sensor 10 based on boththe expected distance and angular orientation offset from a passive loop35.

Each plasma loop sensor 10 has its electrodes connected to atransmitting and receiving circuit (not shown in FIG. 3) with switchingbetween modes and loop sensors 10, such as will be described in moredetail below.

FIG. 4 diagrams one possible transceiver circuit 200 for use with anarray 100 of plasma loop antennas 10 mounted in substrates 5 forprotection during use. A DC power supply 205 is connected to a mixer 210and an analog to digital converter 230. The power supply 205 ispreferably one which provides standard digital and other voltages neededfor operating the circuit components.

The transmit segment 215 of the circuit 200 includes RF CW oscillator210 having its output connected to an RF amplifier 220. The RF amplifier220 combines a CW signal from the oscillator 210 with a modulated signalfrom a connected RF modulator 225 and generates an amplified pulsemodulated (PCM) signal having information for transmitting with theplasma loop sensors 10. The PCM signal is sent to the plasma loop sensorarray 100 for energizing an active one of the plasma loop sensors 10 andcreating a magnetic field and electromagnetic wave.

The PCM signal may be varied using a digital code generator 230connected to the RF modulator to produce different RF modulated signals.The varying PCM signal in turn provides a time-varying signal to theactive plasma loop sensor 10 and results in a time-varying magneticfield and electromagnetic wave being produced by the plasma in theactive plasma loop sensor 10. The digital code generator 230 provides acode word from a look-up table stored in ROM 240. Changing the code wordcauses the RF modulator to produce different RF modulated signals.

The RF amplifier 220 outputs the PCM signal to sensor switch 270connected to plasma loop sensor array 100. Sensor switch 270 controlsswitching between the transmit 215 and receive 235 circuit segments.Preferably, the sensor switch 270 cyclically alternates between transmitand receive modes.

A switch 105 within array 100 is used to sequentially switch power tothe several plasma loop sensors 10 in array 100. Only one plasma loopsensor 10 is made active at one time; the remaining plasma loop sensors10 do not receive any power so that they are effectively renderedelectromagnetically invisible to the active sensor 10 and do not detunethe active sensor 10. While a plasma loop sensor 10 is active, thesensor switch 270 provides at least one transmit/receive cycle for theactive plasma loop sensor 10.

After the sensor switch 270 permits a transmit phase in which the activeplasma loop sensor 10 generates a magnetic field and electromagneticwave, the sensor switch 270 changes to connect the active plasma loopsensor 10 to a receive segment 235 of the transceiver circuit 200.

The receive segment 235 includes a limiter circuit 260 for ensuring thereceived signal from the array is scaled within the operating range of areceiver 265. The limiter circuit 260 protects the receiver 265 fromover-voltage instances in the received signals. The receiver thendemodulates a coded reply RF PCM signal, which can be generated byinteraction of the active plasma loop sensor 10 with a passive loopwithin the near-field range. If necessary, the receiver can also amplifythe received RF PCM signal to ensure proper decoding.

The transceiver circuit 200 includes components for interpreting thereceived signal. The demodulated coded reply signal is sent from thereceiver 265 to a signal processor 255. The signal processor 255conditions the coded reply signal for input into a code comparator 250.When the conditioned reply signal is input at the code comparator 250,the coded reply is compared to known or expected replies stored in alook-up table stored in ROM.

The result obtained by the code comparator 250 is sent to an output 232.The result may be information received from the passive loop or it maybe a null if no passive loop was detected during the transmit/receivecycle.

The output 232 can be connected to any device capable of using thedigital signal from the A/D converter. For example, in grocery scanningsystem, the output 232 may be connected to a cash register to provideprice and item information received from a scanned object in a grocerybag.

While loop sensors wholly composed of plasma tubes are preferred foruse, FIGS. 5A-5C illustrate metal loop sensors 30 having plasma sections31 which are electromagnetically equivalent to the plasma loop sensors10 described above. The metal loop sensors 30 with plasma sections 31are also magnetically invisible to adjacent loops when the plasmasections 31 are deactivated. That is, the plasma sections 310 aresufficiently long that when the ionizing energy is removed from theelectrode terminals 25, 27, the loop circuit is broken so that amagnetic field will not generate a current in the metal loop 30. Sincecurrent cannot flow through the loop 30 except when the gas 15 isionized to form plasma, the metal loop sensor 30 also appearselectromagnetically invisible and does not cause detuning of surroundingsensors 10, 30 when it is inactive.

The plasma sections 31 act like switches for the metal loop sensors 30to activate and deactivate them in the same manner as the plasma loopsensors 10 are activated and deactivated. When power is supplied to theplasma section 31 through leads 20, 22 and electrodes 25, 27, the metalloop sensor 30 is activated and transmits a magnetic field which caninteract with other adjacent loop sensors. The metal loop sensors 30 canbe connected to a circuit such as that shown in FIG. 4 in the samemanner as the plasma loop sensors 10. Arrays of the metal loop sensors30 can be connected, oriented and sequentially switched using the plasmasections 31 in the same manner as the plasma loop sensors 10 describedherein as well.

The plasma section 31 can be as short as a 1° arc segment of the metalloop sensor 30, up to the entire circumference, less a gap forelectrodes, so that it is the same as plasma loop sensor 10. However,when the metal loop sensor 30 embodiment of the loop sensors 10 is used,it is preferred that the plasma section 31 is an arcuate segment betweenabout 1° and 10° long.

In FIG. 5D, a further alternative loop 10 structure is provided in whicha plasma loop 10 has a switch 80 in series. The switch 80 may be anelectromechanical relay switch, a solid state switch or other similarswitch that is electrically changeable between conducting andnon-conducting positions.

FIGS. 6-8 illustrate scanning arrays 100 of plasma loop readers 10supported in rigid substrates 290, 295.

In FIG. 6, a spherical non-magnetic substrate 295 supports an array 100of plasma loop readers 10 on its surface. The substrate 295 is selectedso that it does not interfere with the magnetic fields and electricalproperties of the plasma loop sensors 10. Although non-magneticsubstrates are preferred, it should be understood that ferrite materialsmay be used for the substrate as well.

The terminal leads 20, 22 of each plasma loop sensor 10 are connected toa switching transceiver (not shown in FIG. 6), such as one like thatillustrated in FIG. 4, so that each plasma loop sensor 10 may besequentially activated.

The plasma loop sensors 10 are arranged around the surface of the sphereoriented along many different radii of the sphere. The orientation ofthe plasma loop sensors 10 allows sequential scanning of a broad rangeof angles for corresponding passive loops 35 within the effective rangeof the plasma loop sensors 10. Since the orientations of the plasma loopsensors 10 varies across the surface of the spherical substrate 295, thesubstrate itself does not need to rotate. The sequential activation ofthe plasma loop sensors 10 virtually rotates the scanning angle withoutmoving the substrate 295. Clearly, when the substrate 295 is spherical,a wide range of angles can be scanned for corresponding receiving loopsin objects carrying the receiving loops.

FIG. 7 illustrates another embodiment of the spherical substrate 295having an array 100 of plasma loop readers 10 embedded within thethickness of the substrate 295. The substrate 295 is shown with the tophalf of the sphere removed. As can be seen, the plasma loop readers 10are oriented at different angles along each of several axes of thesphere. The orientations of the plasma loop readers 10 are selected tomaximize the scanning coverage of the array 100. As in FIG. 6, theplasma loop readers 10 are each connected to a switch and transceivercircuit (not shown in FIG. 7) for sequential activation to ensure thereis no electromagnetic interference between plasma loop readers 10 in thearray 100.

In FIG. 8, a cylindrical substrate 290 has an array of plasma loopsensors 10 arranged around the surface of the substrate 290. Thesubstrate is selected to have the same properties as the sphericalsubstrate 295. The cylindrical substrate 290 scans for correspondingreceiving passive loops located around the axis of the cylinder withinthe effective range of the plasma loop sensors 10. The cylindricalsubstrate 290 with the plasma loop sensors 10 mounted only on thesurface is limited compared to the spherical substrate 295 in that onlytwo axes of receiving passive loop orientations can be fully scannedversus three.

However, if the plasma loop sensors 10 are embedded in a cylindricalsubstrate 290 around the surface and oriented rotated about the cylinderradial axis to different angles, then all three axes can be scanned witha sensor array using the cylindrical substrate 290. That is, passiveloops oriented perpendicular to the longitudinal axis of the cylindricalsubstrate 290 could be detected as well.

Arrays 100 of the plasma loop readers 10 can be used in a variety ofscanning applications to detect a receiving passive loop, such as theone shown in FIG. 2.

FIGS. 9-11 depict different scanning applications for arrays of theplasma loop sensors which take advantage of the fact that the arrayitself does not need to move physically to scan a wide range of angles,as discussed above.

In FIG. 9, a checkout lane 54 of a grocery or department store is shownhaving a cart 53 containing packages or bags 33a containing goods.Depending on the circumstances, either the packages or the goods areeach encoded with a unique receiving passive loop (not shown). The lane54 has two counters 55, 55a each having a plasma loop scanner 56, 56alocated vertically at about the level of the bags 33a in the cart 53.Each plasma loop scanner includes an array of plasma loop sensors and aswitching and transceiver circuit for sequentially activating eachsensor in the array to query the goods in the bags 33a. The outputs ofthe transceiver circuits are connected to a cash register 58 for ringingup each unique goods detected in the cart 53 and completing the sale.

The scanners 56, 56a use an array such as the spherical or cylindricalarrays of FIGS. 6-8, or a semi-sphere array which scans the 180° in thelane 54. The semi-sphere array can be created by cutting the sphericalsubstrate 295 in half and using only one half. The arrays are connectedto a transceiver circuit like that of FIG. 4, or another circuit havinga similar function.

When the transceiver of FIG. 4 is used, the ROM 240 provides a look-uptable for identifying each uniquely coded object having a receivingpassive loop that is detected by the scanners 56, 56a. Either of thecash register 58 or the scanners 56, 56a includes a logic circuit orcomputer for determining when the same receiving passive loop isdetected by a subsequently activated plasma loop sensor in the array.The logic circuit or computer ignores the duplicate detection, whilepassing newly detected goods to the cash register 58 for pricing andtotaling the purchase.

The scanner system of FIG. 9 provides a checkout line in which it isunnecessary for a customer to unload the cart 53 for a clerk toindividually scan items in the bags 33a. The contents of the bags 33acan be determined solely by using the scanners 56, 56a. Further,depending on the effective range of the arrays in the scanners 56, 56a,only one of the scanners may be needed. Where the distance across thelane 54 is too great for a scanner 56 from one side to effectivelydetect receiving sensors on the far side of the lane, the second scanner56a can be used as well.

Used in combination with a known debit and credit card terminal 58aconnected to the cash register 58, a single clerk can effectively manageseveral checkout lanes 54 at once, since the checkout is fully automatedexcept when cash or a check is used as payment. Consumers can bag theirgoods as they shop since it is not necessary to remove the items forcheckout, further eliminating wasted checkout time.

FIG. 10 illustrates a toll collection system in which a toll gate 86 isequipped with a scanner 87 connected to a transaction manager 88. Thescanner 87 includes an array of plasma loop readers 10, 30 as in thecheckout lane scanners 56, 56a. The array is used to rapidlysequentially scan for receiving passive loops oriented in a range ofangles on cars 81, 82, 83 passing underneath the toll gate 86.

Each car 81-83 that will use the system is assigned a unique receivingsensor for identifying the car. The transaction manager 88 containslogic programming for determining whether a particular car 81-83 hasbeen scanned already or if it is unique from prior scanned cars. Thetoll gate 86 may contain anti-fraud devices as well, such asweight-triggered checks against whether a receiving passive loop wasdetected or human toll collectors who can monitor the system.

As will be appreciated, the horizontally and vertically orientedscanners described above can be used in wide range of applications wherean object coded with a unique receiving passive loop passes below oradjacent a scanning array of plasma loop sensors. Further, theparticular vertical or horizontal orientation shown in the examples isnot intended to be limiting, as the scanners could be oriented to anyfixed position which is more practical, subject to ensuring the plasmaloop readers in the scanner are oriented to scan the appropriate area.

And, when a unique identification is not required, but merely detection,the receiving passive loop in the object to be detected does not need toinclude a unique code. The scanning array is used to simply detect thepresence of the receiving passive loop and generate an alert, such as ina store security system or another gated area for holding animals orobjects carrying receiving passive loops having a scanner at the gate.

As an example, in another embodiment of a scanning system, FIG. 11 showsa gate 91 having two walls containing scanners 92, 92a connected to analarm system 93. A person 95 has a card 97 or other substrate carrying areceiving passive loop. If the person 95 passes through the gate 91 withthe card 97, the plasma loop sensors in the scanners 92, 92a will detectthe presence of the card 97 by interaction with the passive loop and thealarm system 93 will generate a response, such as shutting the gate 91,sounding a siren or making a light flash. Such a scanning system can beused for ensuring certain persons do not exit a gated area, providedcompliance with carrying the card 510 can be guaranteed.

Alternatively, the card 97 may contain a coded identifier for the person95. The card 97 may have a unique identifier, or simply coded toindicate membership in a group or class. The card 97 can be coded topermit access through some gates 91 without sounding an alarm, whilepassing others will activate the alarm. In such cases the scanners 92,92a and alarm system 93 include a code table for interpreting which card97 is passing the gate 91 and determining the permissions associatedwith the encoding on the card 97 before sounding an alarm or preventingpassage.

In FIG. 12, a further alternative sensor 60 configuration is displayed.The sensor 60 is formed as a cube with sensor loops 10 provided on atleast three panels 62, 64, 66 adjacent one vertex 65 of the cube. Thesensor loops 10 are connected to a switch, such as in the circuit ofFIG. 4, for activating the sensor loops 10 in cyclical succession. Thesensor loops 10 may be controlled by mechanical switches, plasmaswitches or solid state switches. Preferably, the switch is a lowresistance switch. The resistance when the loop 10 is conducting, orclosed, is preferably less than 1 Ohm, while in the open state, theresistance should be high. The open state capacitance can be low.

It should be understood that any one or a combination of the plasma loopsensor 10, metal loop sensor 30 with plasma section 31 or multiple loopplasma sensor 71 can be used in the arrays and scanning systemsdescribed herein.

The loop antennas described herein can be used effectively in bothnear-field and far-field applications, as defined previously, usingmagnetic induction or electromagnetic wave interaction between sensorloops 10 and passive or active sensed loops. And, the loop antennas areuseful as RFID sensors, able to send and receive electromagnetic wavesignals at frequencies including radio frequency up to Terahertz rangefrequencies. That is, each of the sensors described herein as using onlymagnetic induction can also rely instead upon electromagnetic waveinteraction when the sensing unit or other signal generator is properlydriven, so that the sensor system is expanded for use in far-fieldapplications.

For example, in the toll collection system of FIG. 11, RF frequencyelectromagnetic waves may be generated and interact with a uniquereceiving sensor in each car to generate an RF return signal which isinterpreted in the same manner as the signal generated solely bymagnetic induction. A far-field sensor may be preferable in thisapplication in particular to permit higher vehicle speeds and to providemore distance between a vehicle and toll barriers, since a far-fieldsensor will be effective at a greater range.

Further, although the sensed loops 35 are referred to herein as passiveloops, it is envisioned that the sensed loops can be active also, so asto produce their own electromagnetic field. For example, a lithiumbattery source could be connected with the sensed loop and frequencychanging circuit like that shown in FIG. 2 to power the sensed loop andcircuit. The principles of near-field induction and far-fieldelectromagnetic wave interaction are not changed and the plasma loopsensors 10, 30, 71 can still detect the presence or absence of suchactive sensed loops, as well as receive information from the sensedloops.

An alternate reconfigurable antenna, which can be used as the scanningelement of any of the examples of FIGS. 9-11, among other things, willnow be described with reference to FIGS. 13A-17.

FIG. 13A shows an array 310 of linear variable conductive elements 320on a dielectric surface 330. The array 310 of FIG. 13A represents thefoundation of the steerable antennas described herein. The array isconfigurable, by energizing all, none or specific ones of the elements320, to filter selected frequencies of electromagnetic radiation,including in the optical range. It should be noted that elements 320 aredipoles. Feeds (not shown) are provided to each element 320 in the array310 using connectors which are electrically small with respect to thedipole and relevant frequencies.

Depending on the frequency range desired to be affected by the array310, the variable conductive elements 320 are formed by differentstructures. In the RF frequency range, the variable conductive elements320 are a gaseous plasma-containing element, such as a plasma tube. Inthe millimeter infrared or optical region, the variable conductiveelements 320 can be dense gaseous plasma-containing elements orsemiconductor elements. And, in the optical region, the elements arephotonic bandgap crystals. The variable conductive elements 320 arereferred to herein primarily as gaseous plasma-containing elements orplasma tubes, but, unless specifically stated otherwise, are intended toalternately include semiconductor elements or photonic bandgap crystals,depending on the desired affected frequency of the incidentelectromagnetic waves. And, as used herein, plasma tube or plasmaelement is intended to mean an enclosed chamber of any shape containingan ionizable gas for forming a plasma having electrodes for applying anionizing voltage and current.

FIG. 13B illustrates an alternate embodiment of the array 310 of FIG.13A. In FIG. 13B, a second array 312 has slot elements 322 on adielectric substrate 330. Slot elements 322 may also be plasma elements,photonic bandgap crystals or semiconductor elements, depending on thefiltered frequencies.

The arrays 310, 312 of the invention use plasma elements 320, 322 as asubstitute for metal, as depicted in FIGS. 13A-B. When metal is usedinstead for the elements 320, 322 each layer has to be modeled usingnumerical methods and the layers are stacked in such a way to create thedesired filtering. Genetic algorithms are used to determine the stackingneeded for the desired filtering. This is a complicated and numericallyexpensive process.

In contrast, arrays 310, 312 can be tuned to a desired filteringfrequency by varying the density in the plasma elements. This eliminatesmuch of the routine analysis involved in the standard analysis ofconventional structures. The user simply tunes the plasma to get thefiltering desired. Plasma elements 320, 322 offer the possibility ofimproved shielding along with reconfigurability and stealth. The array310 of FIG. 13A, for example, can be made transparent by simply turningthe plasma off.

As the density of the plasma in a plasma element 320 is increased, theplasma skin depth becomes smaller and smaller until the elements 320,322 behave as metallic elements and the elements 320, 322 createfiltering similar to a layer with metallic elements. The spacing betweenadjacent elements 320, 322 should be within one wavelength of thefrequency desired to be affected to ensure the elements 320, 322 willfunction as an array.

The basic mathematical model for these arrays 310, 312 models the plasmaelements 320, 322 as half wavelength and full wavelength dipole elementsin a periodic array 310, 312 on a dielectric substrate 330.Theoretically, Flouquet's Theorem is used to connect the elements.Transmission and reflection characteristics of the arrays 310, 312 ofFIGS. 13A-B are a function of plasma density. Generally, as plasmadensity increases in the elements 320, 322, the arrays 310, 312 willblock transmission and reflect incident electromagnetic waves ofincreasing frequency.

In the array 310, 312 of FIGS. 13A-B, a scattering element 320, 322 isassumed to consist of gaseous plasma contained in a tube. It should benoted that the plasma elements 320, 322 may be divided along theirlengths into segments 322a for the purpose of defining current modes.

The arrays 310, 312 can be designed to be a switchable reflector. Byplacing the elements 320, 322 closer together, a structure is producedwhich acts as a good reflector for sufficiently high frequencies. Areflective array 12, has the same general structure as in FIG. 13B, butwith the elements 322 more densely packed. For this example, the length,diameter, vertical and lateral spacing are 10 cm, 1 cm, 11 cm, and 2 cm,respectively.

The calculated reflectivity for the perfectly conducting case as well asfor several values of the plasma frequency using the values above wasdetermined. For frequencies between 1.8 GHz and 2.2 GHz the array 12operates as a switchable reflector, dependent upon the plasma frequencyin the scattering elements 322. By changing the plasma frequency of theelements 322 from low (about 1.0 GHz) to high (10.0 GHz or more) values,the reflector goes from perfectly transmitting to highly reflecting.

The arrays 310, 312 can function in this manner based on theunderstanding that the current modes induced in the plasma elements 320,322 have the same form but different amplitude from those for a perfectconductor. The reflectivity of the array 310, 312 is directlyproportional to the squared amplitude of the current distributioninduced in the elements 320, 322 by the incident radiation. Based onthis observation, it is clear the reflectivity of a plasma arraystructure can be obtained from that for a perfectly conducting structureby scaling the reflectivity with an appropriately chosen scalingfunction.

The scaling function is defined based on the results of the exactlysolvable model of scattering from an infinitely long partiallyconducting cylinder. The scaling of the current amplitude vs. plasmafrequency in the plasma FSS array is approximated as an isolatedinfinitely long partially conducting cylinder.

The reflectivity for a perfectly conducting array, obtained by thePeriodic Moment Method, is then scaled to obtain the reflectivity of theplasma array vs. plasma frequency. The results of these calculationssupport the concept that switchable filtering behavior can be obtainedwith the use of the plasma array 310, 312 of FIGS. 13A-B.

With respect to FIGS. 13A-B, it should be observed that while the arrays310, 312 have been described as elements 320, 322 supported ondielectric 330, the arrays 310, 312 may be formed in reverse as well.That is, permanent slots may be formed through a variable conductivearea, such as a plasma body, surrounding the slot. The effective size ofthe slot can be changed with respect to electromagnetic waves bymodifying the properties of the variable conductive area surrounding theslot. For example, by switching a plasma body between conducting andnon-conducting states, and/or changing the frequency and plasma densityin the plasma body, the effective size of the slots can be changed.Changing the effective size of the slots permits the array to filterdifferent frequencies.

An example of the utility of this feature is found in connection withradomes, which are conventionally formed as metal shells with bandpassslots tuned for the enclosed radar antenna operating frequency. A radomeis improved by forming the radome structure from the substrate 330 andproviding an array 310, 312 with slots surrounded by variable conductiveregions on the substrate 330. Unlike a conventional radome, the array310, 312 of the invention can include fixed slots in this embodiment,but is also reconfigurable to pass different frequencies electronicallyrather than mechanically. By changing the conductivity of the variableconductive regions surrounding the slots, the effective slot size ischanged, and the radome is “retuned” to a different frequency. Thus, amultiple frequency radar antenna could be housed in a radome formed byan array 310, 312 of the invention.

In a further variation of this embodiment, the dielectric substrate 330could be replaced by a conductive metal substrate. Depending whether thearray 310, 312 is formed by plasma elements or slots surrounded byvariable conductive regions, the result is either a single frequency ortunable frequency bandpass filter. But, in such case, it should beunderstood that the limitations of using conductive metal as thesubstrate will apply to the function of the arrays 310, 312 used aloneor together.

FIGS. 13C and 13D illustrate further embodiments of the arrays 310 inwhich the plasma-containing elements 320 have different configurationsto produce different effects.

FIG. 13C shows an array 314 which can function as a polarizer. Variableconductive scattering elements 324 in the polarizing array 314 arestar-shaped. Polarization on different axes is effected by changing theconductivity of the several spokes 324a-f of each element 324 in thearray 314. By coordinating the conductivities of each spoke 324a-f ofthe several elements 324 in the array 314, a wave passing through thearray can be polarized. More importantly, the polarization of anincident signal can be controllably changed simply by changing theconductivities of the spokes 324a-f.

In FIG. 13D, the array 316 on substrate 330 is composed of variableconductive elements 326 which are sized progressively smaller in eachrow of the array 316. That is, the top row of elements 326 are largest,while the bottom row of elements 326 are the smallest.

An array 316 as shown in FIG. 13D will produce progressive phaseshifting, for example, when the array 316 is positioned ⅛ wavelengthabove a ground plane (not shown). A standing wave is developed betweenthe dielectric substrate 330 and array 316 and the ground plane.Depending on the effective length of the elements forming the array 316,a phase shift is produced which causes the reflection angle to change.By electrically reconfiguring the length of the variable conductiveelements 326 in the array 316, a flat, variable phase shift, steerableantenna is produced having characteristics otherwise similar to aparabolic steerable antenna with fixed phase shifts.

When multiple arrays as shown in FIGS. 13A-D are used in combination,selective filtering and other effects can be produced. Any of the arrays310-316 can be driven by feeds as well to act as a transceiving antenna,rather than simply powered for producing particular effects. Forexample, a driven array 310 of dipoles as in FIG. 13A, can be combinedwith a polarizing array 314 as in FIG. 13C, a bandpass array 310, 312 ofFIG. 13A or 13B and a phase shifting array 316 of FIG. 13D to transmitpolarized electromagnetic waves at selected frequencies in specific,changeable, radial directions. The arrays 310-316 used should all bespaced within one wavelength of the transmitted frequency of each other.Alternatively, as discussed herein, the arrays 310-316 can be combinedfor use with other driven antennas to control their radiation patterns.

While the variable conductive elements 320, 322, 324, 326 illustrated inFIGS. 13A-D are preferably dipoles or the shapes indicated, the arrays310-316 may be formed by elements 320-326 of different geometric shape.Alternate elements may have any antenna or frequency selective surfaceshape, including dipoles, circular dipoles, helicals, circular or squareor other spirals, biconicals, apertures, hexagons, tripods, Jerusalemcrosses, plus-sign crosses, annular rings, gang buster type antennas,tripole elements, anchor elements, star or spoked elements, alphaelements, and gamma elements. The elements may be represented as slotsthrough a substrate surrounded by variable conductive surfaces, orsolely by variable conductive elements supported on a substrate. Theslots may be filled by a dielectric, or simply be open and filled byair.

FIG. 14A shows a steerable antenna 410 of the invention composed of anomnidirectional antenna 400 surrounded by an annular shield 420. Antenna400 is a dipole, and can be a radiating plasma tube, a conventionalmetal dipole antenna, or a biconical plasma antenna for broadbandradiation. Shield 420 is composed of variably conductive elements whichcan be switched between conducting and non-conducting states, and madeto conduct at different frequencies. In one embodiment, the shield 420may be formed by a cylindrical array formed by curling one or more ofany of arrays 310-316 illustrated in FIGS. 13A-D. In a preferredembodiment, illustrated in FIGS. 14B and 14C and discussed in greaterdetail below, the shield 420 is composed of vertically orientedplasma-containing elements 422, such as plasma tube elements. The plasmatubes 422 form a simple array of one row and multiple columnssurrounding the antenna 400. The plasma tubes 422 may be mounted in asubstrate or other electromagnetically transparent material to assistmaintaining their placement.

The configuration of antenna 410 becomes a smart antenna when digitalsignal processing controls the transmission, reflection, and steering ofthe internal omnidirectional antenna 400 radiation using the shield 420to create an antenna lobe in the direction of the signal. Multilobes maybe produced in the case of the transmission and reception of direct andmultipath signals. The shield 420 is opened or made electricallytransparent to the radiation emitted by the omnidirectional antenna 400using controls to switch sections or portions of the shield 420 betweenconducting and non-conducting states, or by electrically reducing thedensity or lowering the frequency of the shield elements 422.

The distance between omnidirectional antenna 400 and plasma shield 420is important, since for given frequencies, the antenna 410 will be moreor less efficient at passing the transmitted frequencies throughapertures in the shield 420. Specifically, the release ofelectromagnetic antenna signals from antenna 400 depends upon theannular plasma shield 420 being positioned at either one wavelength orgreater from the antenna 400, or at distances equal to the wavenumbertimes the radial distance, or kd, to interact with the transmittedsignals effectively. Thus, an electromagnetically effective distancebetween the shield 420 and antenna 400 is one wavelength or greater ofthe transmitted frequencies the shield is intended to act upon, or atdistances corresponding to kd are satisfied, as discussed furtherherein.

It is envisioned that multiple annular plasma shields 420 can bepositioned around the antenna 400 to provide control over transmissionof multiple frequencies. For example, only the shield 420 correspondingto a desired transmission frequency could be opened along a particularradial, while all other frequencies are blocked through that aperture byother shields 420.

FIGS. 14B and 14C illustrate two embodiments of the antenna 410 of FIG.14A. The antenna 410 in each case is comprised of a linearomni-directional antenna 400 surrounded by a cylindrical shell ofconducting plasma elements 422 forming plasma shield 420. Preferably,the plasma shield 420 consists of a series of tubes 422 containing agas, which upon electrification, forms a plasma. Fluorescent lightbulbs, for example, can be used for tubes 422. The plasma is highlyconducting and acts as a reflector for radiation for frequencies belowthe plasma frequency. Thus when all of the tubes 422 surrounding theantenna are electrified and the plasma frequency is sufficiently high,all of the radiation from omnidirectional antenna 400 is trapped insidethe shield 420.

By leaving one or more of the tubes 422 in a non-electrified state orlowering the frequency below the transmission frequency of antenna 400,apertures 424 are formed in the plasma shield 420 which allowtransmission radiation to escape. This is the essence of the plasmawindow-based reconfigurable antenna, or plasma window antenna (PWA). Theapertures 424 can be closed or opened rapidly, on micro-second timescales in the case of plasma, simply by applying and removing voltages.

FIG. 14B shows the configuration when the PWA 410 has seven activeconductors 422 in the shield 420. The following simple geometricconstruction for creating the plasma shield 420 is used. For forming acomplete shield 420, N cylinders 422 are placed with their centers lyingalong a common circle chosen to have the source antenna 400 as itscenter. Some distance from the origin d is selected as the radius. Thedistance can be calculated to produce optimal results for a given PWA410 frequency, but should be within one wavelength to be effective.Then, the circle of radius d is divided into equal segments subtendingthe angles:Ψ₁=2πdNwhere the integer 1 takes on the values −1, 0, 1, . . . N−1. Theapertures 424 are modeled by simply excluding the correspondingcylinders (plasma tube 422) from consideration. Thus, for example, themathematical model of FIG. 14B was generated by first constructing thecomplete shield 420 corresponding to N=8. Then, the illustratedstructure having one aperture 424 was obtained excluding the cylindercorresponding to 1-2 where we have numbered the cylinders assuming theangle to be measured from the positive x-axis (i.e, extending 90° to theright).

In the following analysis, it is convenient to specify the cylinderradius through the use of a dimensionless parameter τ which takes onvalues between zero and unity. More explicitly, the radius of a givencylinder (all cylinder radii assumed to be equal) is given in terms ofthe parameter τ, the distance d of the cylinder to the origin, and thenumber of cylinders needed for the complete shield N by the expression:a−dτ sin(πN)

It should be noted that there is no need to restrict the steerableantenna 410 to configurations of touching conductor cylinders. When theplasma tubes 422 are powered to sufficiently high plasma density thatthe frequency exceeds the transmission frequencies, the size of any gapsbetween the tubes 422 and distance from the omnidirectional antenna 400determine the extent of signal reflection caused by the plasma tubes422. When spaced properly and powered sufficiently, plasma tubes 422produce a perfectly reflective shield 420 that prevents electromagneticsignals from omnidirectional antenna 400 from escaping and transmitting,even when gaps between tubes 422 are present.

As the plasma density, and therefore, the frequency, are decreased, in aparticular plasma tube 422, that tube becomes transparent forelectromagnetic signals generated by the omnidirectional antenna 400.Thus, if a single plasma tube is powered down so as to be transparent toa particular frequency or all frequencies, an electromagnetic signaltransmitting from omnidirectional antenna 400 will be permitted toescape or broadcast along the radials passing through the apertureformed by the transparent plasma tube 422 and any adjacent gaps. Theantenna signal can be steered by simply opening and closing apertures bypowering and unpowering the plasma tubes 422. The amount of radiationreleased will depend in part upon the distance of the plasma tube ringfrom the antenna 400 times the wavenumber of the antenna radiation.

A multi-frequency steerable antenna can be created by adding furtherrings of plasma tubes 422 spaced apart and at radial distances fromantenna 400 to optimally affect particular frequencies. An antenna ofthis configuration permits selectively transmitting specific frequenciesalong specific radials.

As a further expansion of the frequency bandwidth of the antenna, thetransceiving antenna 400 can be a nested antenna. That is, a smaller,higher frequency antenna can be nested inside a larger, lower frequencyantenna. The nested construction is possible especially when usingplasma antennas, as the plasma chambers forming each antenna areseparated from each other and can be individually made active totransmit or receive. Higher frequency signals from the encased antennawill pass through the plasma of the lower frequency antenna. Theindividual antennas making up the nested antenna can be turned on andoff, providing additional control over the transceived frequencies ofthe reconfigurable antenna 410.

And, the nested antenna configuration can also be used to permitsimultaneous transmission and reception by the reconfigurable antenna410. For example, one frequency can be transmitted by one nestedantenna, while a second frequency band is monitored for reception by asecond one of the nested antennas. Multiple antennas beyond two can benested together to transmit and/or receive on other frequencies.

A more complex application of the arrays of FIGS. 13A-D is shown byFIGS. 15A and 15B, in which several of the arrays are arranged instacked layers 810-818. In each case, the layers 810-818 are selected toproduce a particular effect in conjunction with each other on the signalbroadcast through the surrounded antenna 402. The antenna 402 shown is abiconical, center-fed antenna, which type of antenna is particularlyuseful for broadband applications. The biconical antenna 402 ispreferably a plasma-filled cone antenna, so that the advantages gainedthereby are obtained, including the broad frequency range resulting fromdifferent plasma densities along the length of each end of the antenna402. A transceiver 800 is attached to the antenna 402 through a feed forgenerating and interpreting signals transmitted through and receivedfrom antenna 402.

The array layers 810-818 are arranged concentrically around the antenna402, and are spaced within one wavelength of the transmitted signals ofeach other. The optimal spacing between layers, and elements in eachlayer, can be calculated, as with the shield 120 of FIG. 14A, above. Thespacing between antenna 402 and the layers 810-818 is the same as withthe shields 420 of FIGS. 14A-C, above. The layers 810-818 are selectedto produce a particular effect, such as a selective bandpass filter,polarized transmission, phase shifting, and steering the transmittedsignals by using one of the array types of FIGS. 13A-D for each layer810-818. The substrate 330 of each array type used is preferably formedinto a cylinder, so that the array is equidistant from the antenna 402at each radial.

For example, each layer 810-818 may be a frequency filter, such as thearray of FIG. 13A or 13B. Different frequencies can be selectivelyfiltered by choosing different element 320, 322 configurations in thearrays 310, 312 forming the layers 810-818. That is, for higherfrequency filters, more rows and columns of elements 320, 322 should beused in array like that of FIG. 13A or 13B, while lower frequenciesrequire fewer elements 320, 322 to block. Biconical antenna 402 cangenerate several different frequencies due to the changing cross-sectionof the antenna shape.

The frequency filter formed by layers 810-818 can be used to pass orblock particular frequencies within the range affected by the filter onselected radials, while others are permitted to pass. In a preferredarrangement, layer 810 is an array for reflecting, or blocking, thehighest frequencies transmitted or received, while layer 818 is an arrayfor reflecting the lowest frequencies. Layers 812-816 are selected toreflect progressively lower frequencies between those affected by layers810 and 818. It should be appreciated that higher frequencies willcontinue to pass through lower frequency tuned arrays, even when thosearrays are active. But, to pass the lowest frequency signals, all of theshield layers 810-818 must be effectively opened along the desiredradial(s) by making the array elements non-conducting in the windowwhere the low frequency signal is transmitted. When the arrays aresufficiently large, it is possible to control transmission and receptionin both the radial and azimuth axes by creating a window in the shieldlayers 810-818 and sequentially opening and closing the window.

Alternatively, one of the layers 810-818 may be a polarizer or phaseshifter array, such as illustrated by FIGS. 13C and 13D. The shieldlayers 810-818 work in the same manner as above with respect to receivedsignals. Thus, inclusion of a phase shifter array permits reflection andscattering of certain received signals, such as to avoid activedetection of the antenna 402. For example, the layers 810-818 may bedesigned to deflect incident electromagnetic signals atnon-backscattering angles, so as to produce no, or only a very small,radar cross-section. A phase shifter array provides one arrangement forsteering incident signals. A further use of the layers 810-818 andantenna 402 is to act as a repeater station, for propagating a receivedsignal along all or selected radials.

It should be understood as within the scope of this invention that theantenna 400 of FIGS. 14B and 14C or antenna 402 of FIGS. 15A-B can besubstituted for each other, or other antennas may be used. Onealternative antenna configuration which is contemplated combines two ormore antennas in the same manner as the arrays 310-316 which are stackedin layers 810-818. That is, a conventional omnidirectional dipole may besurrounded by a co-axially oriented helical antenna, or a plasmabiconical antenna may consist of two plasma biconical antennas formed tohave one antenna inside the other, in a nested configuration. A greaterrange of different frequencies may be transceived using the nestedantennas or dual biconical antenna by producing a higher plasma densityin the inner antenna and a lower density in the outer antenna. Thehigher frequencies produced in the inner plasma biconical antenna willpass easily through the lower plasma density of the outer biconicalantenna.

In the case of combining a helical antenna co-axial with anotherantenna, such as a dipole, a multi-axis antenna is formed when thefrequencies are properly selected. The helical antenna will transceiveprimarily along radiation lobes oriented extending on the longitudinalaxis of the helix, while an omnidirectional dipole located along thataxis will transceive mainly in a donut shaped region radiallysurrounding the dipole antenna. The frequencies must be selectedsimilarly to the arrays to ensure proper transmission of higherfrequencies through lower ones.

In a further embodiment, the layers 810-818 may consist of transmittingarrays arranged to produce an arbitrary bandwidth antenna. In such case,the layers 810-818 can be used in conjunction with a shield 420 or otherfiltering array 310-316. The transmitted frequency of layer 810 shouldbe the highest and that of layer 818 the lowest. The layers 810-818 maybe turned on and off to produce single and multi-band effects. When usedas transmitters, the layers 810-818 need not be within one wavelength ofthe adjacent layers 810-818, and can be more effective when spacedgreater than one wavelength apart from the adjacent layers 810-818. Suchspacing does not significantly increase the footprint size of thetransmitting antenna in most cases, for example, when used in themillimeter or microwave bands and higher frequencies, such as used bypersonal or portable electronics.

Further, any of the arrays 310, 312, 314, 316 on substrate 330 may bearranged co-planar or bent to have a particular curvature, such as forparabolic reflectors, or into cylinders, as described above. The arrays310-316 may alternatively be arranged on the surfaces of one or moreplanar substrates 330 to form volumetric shapes surrounding an antenna400 other than cylinders, including closed or open end triangles, cubes,pentagons, etc. While it is preferred that the substrates and arraysform the walls of geometric shapes, the arrays may be conformed to anysurface for use, provided the appropriate calculations are done toensure proper location of the elements for the desired purpose.

Resonant waves set up between layers of elements 320-326 as shown inFIGS. 13A-D will cause the reconfiguration in progressive phase shiftingto provide reconfigurable beam steering from an antenna, such as a hornantenna or similar feed.

In a further modification, the reflective shield can include annulartubes stacked perpendicular around the plasma tubes 422, to provideadditional control over the size of aperture created. When specificannular tubes are unpowered in combination with certain plasma tubes422, a transmission window through the reflective shield is formed alonga particular radial and at a particular elevation. Thus, steering in thevertical direction can be combined with radial steering.

Further, the powered plasma tubes in any cylinder may act as a parabolicreflector for the affected frequencies, thereby strengthening thetransmitted signal through an aperture. Similarly, the plasma densitiescan be adjusted to produce plasma lenses for focusing the transmittedantenna signal beam.

Preferably, the apertures will be at least one wavelength in arc lengthto permit effective transmission. It should be noted that Fabry-PerotEtalon effects may occur for the release of electromagnetic radiationthrough the antenna while powering the plasma tubes 422, but at lowerplasma densities than for signal reflection.

FIGS. 16 and 17 illustrate transmission radiation lobes which can beproduced using the antenna 410 of the invention. FIG. 16 shows how thereflective shield 420 can include a layer of annular plasma tubes 426oriented perpendicular to vertical shield elements. Thus, in FIG. 16, atransmission radiation lobe 430 is produced along a particular radialand at an elevation selected by unpowering the upper ones of the annularplasma tubes 426.

Similarly, in FIG. 17, two different transmission radiation lobes 430are produced by creating apertures on each side of antenna 410 and atdifferent elevations. The transmission radiation lobes 430 illustratedhave side lobes 430a.

The steerable antennas illustrated in each of FIGS. 14A-17 can besubstituted for the loop sensors 10, 30, 71 in each of the examplesabove. The antennas described in FIGS. 14A-17 are particularly useful infar field applications, where the tags which are being sensed are likelylocated outside of an effective near field range. While the loop sensors10, 30, 71 can be used in far field applications as electromagnetic wavetransceivers, they are preferred for use in near-field applications, andthe steerable antennas of FIGS. 14A-17 are preferably used in far fieldapplication.

FIG. 18 illustrates how the steerable antennas can be used in a scanner850 to scan an area for ID tags 900. The ID tags 900 can be both passiveand active, or activatable ID tags 900 as will be further described. Thescanner 850 consists of reconfigurable antenna 410 and transceiver 800.The reconfigurable antenna 410 of scanner 850 emits a radiation lobe 430through an opened section of the antenna shield 420 (not shown in FIG.18). The radiation lobe 430 interacts with the ID tags 900 to sensetheir presence, or read data from the tags 900, and, in some cases,write date to the tags 900 as well. The radiation lobe 430 can be madeto sweep a full circle around the antenna 410 by controlling whichradials of the shield are opened and closed, so that scanning isintentionally limited to a single direction at a time, even though theactual transceiving antenna used in reconfigurable antenna 410 is anomnidirectional antenna. Transceiver 800 may contain switching andcontrol programs for operating the shield and reconfigurable antenna 410to this end.

Alternatively, the transceiver 800 may be two distinct units connectedto different antennas within reconfigurable antenna 410. For example,the reconfigurable antenna 410 may use plasma nested antennas, stackedarrays, and plasma shields around an omnidirectional antenna as plasmafilters or plasma frequency selective surfaces as individual layers ortwo or more layers to create large bandwidths or multi-bandwidthradiation patterns, so that one antenna transmits while the otherreceives, and no switching is necessary to control the transceiver 800.The arrangement permits simultaneous transmission and reception ofsignals, and the antenna 410 can operate continuously, if desired. Theshield 420 still must be controlled to adjust the radial on which theantenna 410 transmits and receives simultaneously.

The ID tags 910, 920, 930 in FIG. 18 represent different versions oftags which can be sensed by the antenna 410. ID tags 930 are simply anytype of antenna capable of interaction with the scanner 850 operatingfrequency. For example, ID tags 930 can be conductive metal loops, orother known RFID tags.

ID tags 910 and 920 are more complex versions which include an antenna900, a code 902 and a power source 905, 907. The code 902 is connectedwith and transmitted by antenna 900 so that ID tag 910, 920 can providemore information to scanner 850 than simply indicating its presence, aswith tags 930. The power sources 905, 907 operate differently, dependingon the type of ID tag 910, 920.

ID tag 920 is shown in the active state, in which it transmits a tagradiation lobe 908 that interacts with the scanner radiation lobe 430.ID tag 920 is continuously powered by power source 905, so that itcontinuously generates radiation lobe 908. Power source 905 may be abattery sufficient to power antenna 900 or other power source withsimilar ability. Code 902 can include a controller for switching thepower source 905 on and off, for example, when antenna 900 is a plasmaloop 10, 30, 71, and a memory for storing an identifier and possibly forreceiving and writing data transmitted by a scanning signal. ID tag 920thus has two states—on and off. In the off state, it iselectromagnetically invisible to the scanner 850 and cannot be activatedwithout application of significant external power. In the on state, IDtag 920 actively provides information to scanner 850.

ID tag 910 represents yet another embodiment in which the antenna 900 isa plasma loop 10, 30, 71 that is weakly ionized or weakly powered bypower source 907. The power source 907 may be a radioactive seed, a weakbattery or other voltage source, or other known power sources. Whenscanner radiation lobe 430 impinges on antenna 900, the powertransmitted by scanner 850 is sufficient to activate plasma loop 10, 30,71 so that code 902 can be read by the scanner 850. One scanning antennasuitable for energizing the activatable ID tags uses pure neon gasplasma with a mercury additive. The antenna produces a plasma with highcurrent at about 6 Torr pressure, without requiring a significant powerincrease to the scanning antenna.

Alternatively, the antenna 900 may be activated by an external powersource other than the radiation lobe 430. In the weakly ionized state,ID tag 910 is electromagnetically invisible and does not interfere withother devices.

It should be noted that both ID tags 910 and 920 can be provided with orwithout code 902. Thus, the ID tags of the invention may be active orinactive transmitting tags (have a code 902), active or inactive passivetags (no code 902—sensed by interference only), and active or inactiveactivatable tags (have a weakly powered plasma antenna, with or withoutcode 902).

FIG. 19 demonstrates a further application of the antennas describedherein used in a scanner for determining the contents of a ship 1000entering a port or at dockside 990. The scanner again consists of anantenna 410 like that of FIGS. 14A-17 and a transceiver. The antenna 410is mounted to a tower or building 980, which may include a control roomfor monitoring the scanning. The scanner transmits along a radiationlobe 430 in a direction selected by configuration of the antenna 410. Inone embodiment, the radiation lobe 430 may be kept fixed, for example,as the ship sails past the antenna 410. Alternatively, the radiationlobe 430 can be swept through between angles parallel to the dock 990and crossing all of the containers 950 on the ship 1000. In such case,the ship 1000 can remain stationary or move past the antenna 410.

In order for the scanner to be effective, a modification must be made toconventional shipping containers 950 to permit electromagnetic radiationto penetrate the container. The walls of the containers must have slots960, similar to those used in arrays 310-316. The slots 960 are formed,for example, by dielectrics in the metal sides, which permits thescanning signals to interact with ID tags 900 on goods in the containers950. The slot 960 configuration in the container 950 walls willdetermine what bandwidth of scanning frequencies can be used effectivelyto read and/or write to ID tags 900 on the container contents.

Further, it is envisioned that the interiors of the containers will belined with electromagnetic absorptive material or absorbing dielectriccones. Such interior lining will prevent resonant signals from buildingwithin the container and causing unwanted interference with the scanningsignal. As a further alternative, the ship hull, or when applied onland, a truck or airplane body, can be formed with dielectric slots forpermitting specific frequencies to penetrate the hull and scan thecontents for ID tags. The dielectric slots may be formed as describedherein in connection with the arrays 310-316 as well. That is, the slotscan be variable dielectric slots formed by variable conductive elementswhich either permit or block electromagnetic waves from passing, slotssurrounded by variable conductive regions, or a constant dielectricmaterial selected and arranged to permit a particular frequency band topass.

While the example of FIG. 19 is described using the plasma windowantenna 410, it should be understood that any of the reconfigurableantennas disclosed herein could be used. The same scanning can be doneusing the plasma loop sensors 10, 30, 71 in near or far field operation,as the distance between antennas and ID tags requires.

The scanners disclosed herein in each of the examples of FIGS. 9-11, 18and 19 can use any of the antennas disclosed as the scanning element.That is any scanner disclosed can have plasma window antenna 410,stacked arrays 810-818, or arrays of plasma loop sensors 10, 30, 71 asthe scanning element which broadcasts the scanning signal connected to atransceiver or similar component. Whichever antenna type is selected asthe scanning element, a radiation lobe is generated based on informationfrom the transceiver 800 for interaction with ID tags in the effectiverange of the scanner. Thus, while multiple plasma loops 10, 30, 71 aresequentially activated to scan multiple directions in one embodiment,the same scanning can be done using the plasma window antenna 410 bysequentially opening a transmission window to direct the transmissionlobe along selected radials.

In all of the applications discussed above, plasma-containing elementsused as plasma antennas or passive plasma elements can be operated inthe continuous mode or pulsed mode. During the pulse mode, the plasmaantenna or passive plasma elements can operate during the pulse, orafter the pulse in the after-glow mode. To reduce plasma noise, theplasma can be pulsed in consecutive amplitudes of equal and oppositesign. Phase noise can be reduced by determining whether the phasevariations are random or discrete and using digital signal processing.Phase noise, thermal noise, and shot noise in the plasma can also bereduced by digital signal processing.

It is recommended that AC bipolar pulses operated at a frequency abovethe ion acoustic wave frequency in the plasma be applied to the plasmafor ionization and transmission purposes be used so as to reduce noisein any of these plasma antenna systems, including plasma antennas,plasma arrays including stacked plasma arrays both active and passive,plasma nested antennas, plasma shields, and any plasma readers or plasmaantenna tags both active and passive. During the pulse cycle, the timebetween pulses called the afterglow state is the least noisy state.

All of the plasma elements described herein can be operated in theafterglow state using AC bipolar pulses with frequencies above the ionacoustic wave frequencies to minimize noise. This technique also reducespower requirements for the plasma elements. To maximize the amount oftime the plasma antenna or plasma shields are in the low noise afterglowregion, the pulse width in time should be minimized and the time betweenpulses should be maximized. During the pulse, the electron beam from theelectrodes in the plasma tube containing the plasma can transfer energyinto waves in the plasma which in time create nonlinearities and noise.Some of these waves are at or near the plasma frequency. Some of thesewaves are in the range of between 2 KHz to 15 KHz, which are in therange of ion acoustic waves. Much of the noise created by the transferof energy from the electron beam from the electrode to waves in theplasma can be controlled by controlling the electron beam. In practicethe amount of energy from the electron beam feeding these waves can becontrolled by chopping the electron beam and creating a pulse.

Other designs that can reduce noise in the plasma include providingelectrodes with enough energy spread or energy jitter to reduce thetransfer of energy from the electron beam from the electrodes to thewaves in the plasma. Still other ways of controlling the noise in theplasma include using plasma antennas or plasma tubes without electrodesfor any of the plasma elements. Mechanisms for coupling energy into theplasma if electrodes are not used include capacitive sleeves around theplasma tubes, inductive couplers into the plasma tubes, or remoteionization. Remote ionization can be achieved by lasers, other antennas,acoustics, or other means.

Each of the scanners described above can be mounted within a suitablecasing for permitting the antennas to operate as described. It isenvisioned as well that the components making up the antennas of eachscanner can be embedded within a material having a dielectric constantwhich approximates air. For example, a synthetic foam including a largevolume of air bubbles used to support the antenna elements can have adielectric constant which approximates that of air. That is, the plasmaloops or reconfigurable antenna can be held in place by a rigid,air-filled foam. The foam can further be formed to have external cones,like those used in an electromagnetic anechoic chamber, which reducereflection. When such a supporting structure is used, the scanner can befully encased and protected from damage, but still operate normally, asthe casing material does not adversely affect the ability of theantennas to function. Other materials having similar properties can beused, while those with different dielectric constants can also be used,but are less preferred due to their adverse affect on signal strength.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

1. A reconfigurable scanner for scanning for ID tags containingscannable antennas oriented in multiple directions relative to thescanner, without need for physical movement of the scanner, thereconfigurable scanner comprising: a scanning element broadcasting asignal in a selected direction, the scanning element having a pluralityof variable conductive elements; control means for electricallycontrolling and changing the selected direction in which the scanningelement broadcasts the signal by powering and unpowering the pluralityof variable conductive elements; and transceiver means for generating anelectromagnetic wave and receiving a responsive electromagnetic wavesignal from a sensed ID tag within an effective range of the scanner,whereby unpowered variable conductive elements do not cause anyinterference with the scanning signal.
 2. A reconfigurable scanneraccording to claim 1, wherein the plurality of variable conductiveelements are a plurality of plasma loop sensors.
 3. A reconfigurablescanner according to claim 2, wherein the plasma loop sensors eachcomprise a loop antenna having at least a portion of which is an arcuatetube section containing an ionizable gas, such that the loop antenna isonly conductive when the ionizable gas is ionized.
 4. A reconfigurablescanner according to claim 1, wherein the scanning element comprises anantenna and an electromagnetic shield formed by the plurality ofvariable conductive elements, the electromagnetic shield intersectingtransmission lobes of the antenna in at least the multiple directionsbeing scanned.
 5. A reconfigurable scanner according to claim 4, whereinthe plurality of variable conductive elements are mounted in an array ona substrate forming the shield.
 6. A reconfigurable scanner according toclaim 5, wherein the substrate is a conductive metal.
 7. Areconfigurable scanner according to claim 4, wherein the electromagneticshield is formed by stacked layers of arrays of the variable conductiveelements.
 8. A scanner system comprising: a plurality ofelectromagnetically scannable ID tags; and a reconfigurable scannerhaving a scanning element with a plurality of variable conductiveelements switchable between electromagnetically active andelectromagnetically invisible, control means for switching the variableconductive elements between electromagnetically active andelectromagnetically invisible, and a transceiver means for generatingand receiving an electromagnetic scanning signal in a directiondetermined by the control means, the scanning signal interacting withthe scannable ID tags located in the direction of the scanning signal.9. A scanner system according to claim 8, wherein the variableconductive elements are plasma loop sensors.
 10. A scanner systemaccording to claim 8, wherein the scanning element comprises an antennaand an electromagnetic shield formed by the plurality of variableconductive elements, the electromagnetic shield intersectingtransmission lobes of the antenna in at least the multiple directionsbeing scanned.
 11. A scanner system according to claim 10, wherein theplurality of variable conductive elements are mounted in an array on asubstrate forming the shield.
 12. A scanner system according to claim11, wherein the substrate is a conductive metal.
 13. A scanner systemaccording to claim 10, wherein the electromagnetic shield is formed bystacked layers of arrays of the variable conductive elements.
 14. Ascanner system according to claim 8, wherein at least one of theplurality of ID tags comprise an antenna and a code connected with theantenna for detection and reading by the reconfigurable scanner.
 15. Ascanner system according to claim 14, wherein the at least one ID tagfurther comprises a power source for powering the antenna into an activestate.
 16. A scanner system according to claim 15, wherein the powersource is external of the ID tag.
 17. A scanner system according toclaim 14, wherein the antenna of the at least one ID tag is a plasmaloop, and the at least one ID tag further comprises a power source forweakly or partially ionizing a plasma in the plasma loop, whereby theplasma loop remains electromagnetically invisible until external energyis received by the plasma.
 18. A scanner system according to claim 17,wherein the external energy is provided by the scanning signal.
 19. Ascanner system for detecting the contents of a shipping container, thesystem comprising: a plurality of slots formed in the shipping containerfor permitting a selected bandwidth of electromagnetic wave to penetratethe shipping container; at least one electromagnetically scannable IDtag associated with the contents of the shipping container; and areconfigurable scanner having a scanning element with a plurality ofvariable conductive elements switchable between electromagneticallyactive and electromagnetically invisible, control means for switchingthe variable conductive elements between electromagnetically active andelectromagnetically invisible, and a transceiver means for generatingand receiving an electromagnetic scanning signal, the scanning signalhaving a frequency within the selected bandwidth for penetrating theshipping container to detect the at least one ID tag.
 20. A scannersystem according to claim 19, wherein the shipping container comprisesdielectrics on the interior of the container for damping resonantsignals.
 21. A scanner system according to claim 19, wherein the slotsare formed by dielectric materials.
 22. A scanner system according toclaim 19, wherein the slots are formed by one of variable dielectricmaterials surrounded by conductive material and fixed dielectricmaterials surrounded by variable conductive material.
 23. Areconfigurable scanner for scanning for ID tags containing scannableantennas oriented in multiple directions relative to the scanner,without need for physical movement of the scanner, the reconfigurablescanner comprising: a scanning element broadcasting a signal in aselected direction, the scanning element having a plurality of variableconductive elements; a switch that electrically controls and changes theselected direction in which the scanning element broadcasts the signalby powering and unpowering the plurality of variable conductiveelements; and a transceiver that generates an electromagnetic wave andreceives a responsive electromagnetic wave signal from a sensed ID tagwithin an effective range of the scanner, whereby unpowered variableconductive elements do not cause any interference with the scanningelement.
 24. A reconfigurable scanner according to claim 23 wherein theplurality of variable conductive elements are a plurality of plasma loopsensors.
 25. A reconfigurable scanner according to claim 24 wherein theplasma loop sensors each comprise a loop antenna having at least aportion of which is an arcuate tube section containing an ionizable gas,such that the loop antenna is only conductive when the ionizable gas isionized.
 26. A reconfigurable scanner according to claim 23 wherein thescanning element comprises an antenna and an electromagnetic shieldformed by the plurality of variable conductive elements, theelectromagnetic shield intersecting transmission lobes of the antenna inat least the multiple directions being scanned.
 27. A reconfigurablescanner according to claim 26 wherein the plurality of variableconductive elements are mounted in an array on a substrate forming theshield.
 28. A reconfigurable scanner according to claim 26 wherein theelectromagnetic shield is formed by stacked layers of arrays of thevariable conductive elements.
 29. A reconfigurable scanner according toclaim 23 wherein said plurality of variable conductive elements are ofat least two dimensional configurations.
 30. A steerable antenna fordirecting the sensitivity of a scanner that scans for ID tags, saidsteerable antenna comprising: an omnidirectional antenna having a firstaxis, an annular shield having a longitudinal axis parallel to saidfirst axis, said annular shield positioned a selected distance from saidfirst axis, said annular shield having a plurality of elements, each oneof said plurality of elements including a plasma element that isvariably conductive between a conducting state and a non-conductingstate, said annular shield allowing passage of a signal in a selectedradial direction relative to said first axis; and a switch configured tocontrol each one of said plurality of elements between said conductingstate and said non-conducting state.
 31. The steerable antenna of claim30 wherein each said plasma tube has a plasma density sufficient toreflect a signal at a selected frequency, said omnidirectional antennaoperable at said selected frequency.
 32. The steerable antenna of claim30 wherein each said plasma tube has a plasma density less than thatnecessary to reflect a signal at a selected frequency, saidomnidirectional antenna operable at said selected frequency.
 33. Thesteerable antenna of claim 30 wherein each said plasma tube has a plasmadensity less than that necessary to reflect a signal at a selectedfrequency and said switch operates at a speed wherein said selectedradial direction is changeable between a first and a second radialdirection in less than a millisecond.
 34. The steerable antenna of claim30 wherein said selected distance is a multiple of one wavelength of asignal to which said omnidirectional antenna is responsive.
 35. Thesteerable antenna of claim 30 wherein said selected distance is greaterthan one wavelength of a signal to which said omnidirectional antenna isresponsive.
 36. The steerable antenna of claim 30 wherein said pluralityof elements are individually made to conduct at a plurality offrequencies.
 37. The steerable antenna of claim 30 wherein each one ofsaid plurality of elements is substantially parallel to said first axis.38. The steerable antenna of claim 30 wherein each one of said pluralityof elements is a ring that encircles said first axis, said rings beingstacked to form a cylindrical shape.
 39. The steerable antenna of claim30 wherein said annular shield has a substantially cylindrical shape.40. The steerable antenna of claim 30 wherein said annular shield has asubstantially spherical shape.
 41. The steerable antenna of claim 30wherein said plurality of elements includes a first group of elementsand a second group of elements, said first group of elements having asubstantially cylindrical shape, said second group of elements having asubstantially cylindrical shape, said first group of elements positionedinside said second group of elements.
 42. A pulsed scanner for scanningfor ID tags containing scannable antennas, said pulsed scannercomprising: a scanning element responsive to a signal having a selectedbearing, said scanning element having a plurality of variable conductiveelements; a switch configured to electrically control and change adirection to which said scanning element is responsive by powering andunpowering selected ones of said plurality of variable conductiveelements, each one of said plurality of variable conductive elementsincluding a plasma element; and a transceiver connected to said scanningelement, said transceiver configured to generate an electromagnetic wavesignal and receive a responsive electromagnetic wave signal, saidresponsive electromagnetic wave signal received by one of said pluralityof variable conductive elements when said plasma element is in anafterglow state.
 43. The pulsed scanner of claim 42 wherein said plasmaelement includes an energized state and said afterglow state, saidenergized state resulting from an ac bipolar pulse applied to saidplasma element.
 44. The pulsed scanner of claim 43 wherein said acbipolar pulse has a frequency greater than an ion acoustic wavefrequency.
 45. A pulsed scanner for scanning for ID tags containingscannable antennas, said pulsed scanner comprising: a scanning elementreceiving a signal from a selected direction, said scanning elementhaving a plurality of variable conductive elements; a switch thatelectrically controls and changes a direction to which said scanningelement is responsive by powering and unpowering selected ones of saidplurality of variable conductive elements, each one of said plurality ofvariable conductive elements including a plasma element, each saidplasma element being powered by a pulse, said pulse being an ac bipolarpulse; and a transceiver connected to said scanning element, saidtransceiver configured to generate an electromagnetic wave signal andreceive a responsive electromagnetic wave signal.
 46. The pulsed scannerof claim 45 wherein said ac bipolar pulse has a frequency greater thanan ion acoustic wave frequency.